The invention concerns generally the electromechanical structures of keyboards. Especially the invention concerns the integration and coupling of keyboard structures into the other electrical and mechanical parts of an electronic device.
A keypad consisting of individually pressable numeric or alphanumeric keys as well as function and control keys is an important part of many modern electronic devices. Especially keyboards of this kind appear in portable telecommunication devices such as mobile telephones and multi-function communicators, portable digital assistants and palmtop computers. FIG. 1 is an exploded cross-sectional view that shows a known structural arrangement which is built on a printed circuit board or PCB 101 with a number of contact pads on its upper surface. An insulating film 102 covers the PCB except of those contact pads onto which components are to be placed. The keyboard part comprises a dome sheet 103 (or alternatively a selection of individual key domes) made of some resilient material that is electrically conductive at least at the concave surface of each dome. Additionally the keyboard part comprises a rubbery keymat 104 where the bulging protrusions coincide with the conductive domes of the dome sheet 103 and constitute the visible parts of the keys. A microphone 105 is coupled to its contact pad(s), and the keyboard-microphone entity is covered by an ESD shield 106 which is made of a thin sheet of stainless steel or other conductive material.
The electric implementations of keypads come in various forms. FIG. 2 illustrates a known rectangular keypad that consists of Nxc3x97M keys; here N=4 and M=5. The arrangement comprises N conductive input rows 201 to 204 and M conductive output columns 205 to 209 where normally the rows and columns do not have a galvanic contact with each other. At each junction of a row and a column there is a key comprising a conductive dome shown as a dotted circle. Pressing for example the key shown as 210 will cause the conductive dome to produce a galvanic contact between row 204 and column 209. An input voltage distributor 211 energises each of the input rows in cyclical turns at a rate that is expected to exceed the rate of pressing the keys by the user. An output detector 212 detects the energisation caused in one of the output lines when the energisation of the corresponding input line coincides with a conductive dome coupling the input and output lines at one intersection. The logical encoding block 213 converts the observation of a keystroke to a bit sequence that unequivocally indicates, which key was pressed.
The drawbacks of the arrangement of FIG. 2 comprise the relatively high power dissipation of the N parallel voltage buffers included in block 211 that are required to maintain the continuous sequential energisation of the input rows.
From the European patent publication number EP 695 041, which is incorporated herein by reference, there is known a keypad arrangement according to FIG. 3. A supply voltage Vcc is provided to a positive voltage rail 301 through a resistor 302. From the positive voltage rail there is a connection to each column of keys through a resistor 303-306 so that each of the resistors has a different value. Each row of keys is coupled to a ground rail 307 so that from each row there is a connection to a common connection point through a differently valued resistor 308-312, and from the common connection point there is a connection to the ground rail through a resistor 313. The positive voltage rail 301 is coupled to a first output port 314. The circuit comprises also a PNP transistor 315 the emitter of which is coupled to the supply voltage Vcc, the base of which is coupled to the positive voltage rail 301 through a resistor 316 and the collector of which is coupled to the ground rail 307 through a resistor 317. The collector of the PNP transistor 314 is also coupled to a second output port 318.
The idea of the keypad according to FIG. 3 is that when a certain key is pressed, a resistive current path is produced between the positive voltage rail and the ground rail. The resistance value of the current path is unambiguously associated to the depressed key, because all the column resistors 303-306 and row resistors 308-312 have different values. The variable resistance of the current path acts as a part of a voltage divider together with the resistor 302, which in turn causes the positive voltage rail to assume a voltage value that is also unambiguously associated to the key which was pressed. An A/D converter (not shown) connected to the first output port converts the voltage value of the positive voltage rail into a digital word representing the depressed key. The circuit built around the PNP transistor will generate an interrupt signal that is readable from the second output port and indicates when a key is pressed.
From the European patent application number 99660183.7, which is incorporated herein by reference, there is known a keypad arrangement the functional principle of which is illustrated in FIG. 4. The arrangement comprises a first resistive strip 401 and a second resistive strip 402 in a parallel, non-touching set-up. At point 403 there is a temporary conductive connection between the resistive strips, caused by for example a conductive dome being pressed simultaneously against the surfaces of both resistive strips. The first end (the left-hand end in FIG. 4) of the first resistive strip 401 is coupled to a supply voltage Vcc through a resistor R1 and the second end of the resistive strip 401 is coupled to the ground potential. Correspondingly the first end of the second resistive strip 402 is coupled to an output port for an output voltage Vout to be measurable and the second end of the second resistive strip 402 is not coupled to anywhere.
When one knows the specific resistances of the resistive strips 401 and 402 and the distances X and Y, it is possible to calculate the resistance values of the pieces of resistive strips on each sides of the connection point. By assuming that the output port where the output voltage Vout is measured sinks only a negligible current, it is possible to further calculate the output voltage Vout which turns out to depend only on the position of the conductive connection 403 in relation to the ends of the first resistive strip, as well as on the specific resistance of the strip. Detection of individual key presses boils down to the task of measuring an analogue voltage value. The application EP 99660183.7 suggests various meandering and other shapes to the resistive strips on the surface of a printed circuit board or the like.
As long as the electrical implementations of FIGS. 2, 3 and 4 are based on the mechanical structure shown in FIG. 1, they all share the disadvantage of requiring a relatively large area from the printed circuit board. Additionally they tend to impose limitations to the layout design of the printed circuit board even though the principle shown in FIG. 4 allows for considerably more flexible implementations in this respect than the other solutions.
It is an object of the invention to provide a keypad structure which is advantageous in terms of manufacturing and places only few limitations to the layout design of both the printed circuit board and the appearance of the keypad itself. It is also an object of the invention to provide a keypad structure that allows reliable and accurate detection of key presses and is tolerable to statistical variation in component characteristics and changes in environmental conditions.
The objects of the invention are achieved by building the keypad as a part of a cover part and by using a suitable coupling to realize the electrical connections between the keypad and other electronic parts of the device in question.
The keypad arrangement according to the invention comprises
a keypad comprising activatable keys,
input signal means for bringing an electric input signal to the keypad,
signal conversion means for responding to a key activation by selectively converting the input signal into an electric output signal the value of which depends on the activated key and
output signal detection means for detecting the value of the electric output signal;
it is characterized in that it comprises a first electromagnetic coupling between the input signal means and the signal conversion means, and a second electromagnetic coupling between the signal conversion means and the output signal detection means.
The invention applies also to an exchangeable cover for a mobile telecommunication device, comprising a keypad that comprises activatable keys; it is characterized in that the exchangeable cover comprises
first electromagnetic coupling means for electromagnetically obtaining an electric input signal to the keypad from an external input signal source,
second electromagnetic coupling means for electromagnetically providing an output signal from the keypad to an external signal detector and
signal conversion means for responding to a key activation by selectively converting an electric input signal into an electric output signal the value of which depends on the activated key.
Additionally the invention applies to a set of exchangeable covers for a mobile telecommunication device, comprising
a first cover and a second cover
within each of said first and second covers means for mechanically attaching the cover to the same mobile telecommunication device,
within each of said first and second covers a keypad that comprises activatable keys;
it is characterized in that
the first cover and the second cover each comprise first electromagnetic coupling means for electromagnetically obtaining an electric input signal to the keypad from an external input signal source,
the first cover and the second cover each comprise second electromagnetic coupling means for electromagnetically providing an output signal from the keypad to an external signal detector,
the first cover and the second cover each comprise signal conversion means for responding to a key activation by selectively converting an electric input signal into an electric output signal the value of which depends on the activated key and
the first cover has the keys of the keypad at different locations than the second cover.
Additionally the invention applies to an electronic device comprising
a keypad that comprises activatable keys,
an outer cover part,
a printed circuit board,
input signal generating means for generating an input signal to the keypad,
signal conversion means for responding to a key activation by selectively converting the input signal into an output signal the value of which depends on the activated key and
output signal detection means for detecting the value of the output signal;
it is characterized in that
the input signal generating means and the output signal detection means are located within the printed circuit board,
the signal conversion means are attached to the outer cover part and
the electronic device comprises a first electromagnetic coupling between the input signal generating means and the signal conversion means, and a second electromagnetic coupling between the signal conversion means and the output signal detection means.
According to the invention a majority of the electromechanical parts of the invention are structurally associated with a cover part and not with an xe2x80x9cengine PCBxe2x80x9d which is a common designator of such printed circuit boards that comprise a remarkable number of electronic components that implement the main functionalities of an electronic device. A capacitive coupling is regarded as the most advantageous form of electrically coupling the keypad to the other electronic parts of the device. The basic form of such a capacitive coupling is a pair of conductive electrodes of which a first electrode is connected to the electric parts of the keypad and a second electrode is connected to the circuitry on an engine PCB. Attaching the cover part and the PCB together brings the electrodes very close to each other so that together they constitute a coupling capacitor.
As a consequence of the invention numerous advantages are achieved. The threedimensional design of the keypad is freed from the planarity of printed circuit boards. The location, number and size of keys becomes completely independent of the PCB layout. Large areas of PCB surface space are left free for other use. Realization of watertight or even hermetically sealed keypad-controlled devices becomes easier. Key press indications can be transmitted through movable joints between device parts. A great degree of freedom is given to the design of exchangeable outer covers.